Three way join mapping

Question

I have three tables file, file_location, and file_object as bellow:
file_location (file_id, file_system_id, stored_on,...)
file (file_id, name, object_id, file_size, ...)
file_object (object_id, created_on, ...)
I need to issue a query that do three way join, for example
SELECT floc.stored_on, mf.file_size, mo.created_on
FROM file_location floc, file mf, file_object mo
WHERE mf.file_id = floc.file_id
AND mo.object_id = mf.object_id
AND floc.file_system_id = 1
How can I avoid doing a separate query for each File and each FileObject(without writing custom SQL to produce some new view-object)?
I would expect the tool to be able to do this in three queries, one for each table, which are then linked up on the client side.
Does anybody know how to do this? I tried different approach including use join, but always end up with separated queries for each object.
Thanks

Answer 1

I'm am not clear on your object model, or the relations between File, FileObject and FileLocation, but in general to optimize queries in TopLink you can use batch reading, joining, partial object reading or report queries.
If you just want the three data values back, one from each object, you might be best off using a ReportQuery.
i.e.
ExpressionBuilder file = new ExpressionBuilder();
ReportQuery query = new ReportQuery(File.class, file);
query.addAttribute("fileSize");
query.addAttribute("storedOn", file.get("location").get("storedOn"));
query.addAttribute("createdOn", file.get("fileObject").get("createdOn"));
List result = (List) session.executeQuery(query);
To read the objects, assuming File has a 1-1 to FileObject and 1-1 to FileLocation, you could query for File and batch read FileObject and FileLocation.
i.e.
ReadAllQuery query = new ReadAllQuery(File.class);
query.addBatchReadAttribute("fileObject");
query.addBatchReadAttribute("fileLocation");
List result = (List) session.executeQuery(query);
If your are trying joining/batch reading and still getting separate calls please include your query definition. It could be that you have already read and cached these objects with value holders in them, or are not using indirection in general.